The field of the invention involves particle counters for analyzing blood cells and platelets suspended in liquid media.
Three methods presently exist for the counting of blood cells. These include: (1) manually counting the cells in a known dilution by use of a microscope; (2) semi-automated systems, in which proper dilutions are first prepared independently, and the dilutions are then introduced to an instrument for electronic cell counting; (3) fully automated systems wherein the blood sample is directly introduced into an instrument which performs all required functions, including dilutions and counts. The first has the disadvantage of being slow, laborious, and subject to human error in counting. The third had the disadvantage of being very expensive.
Automatic and semi-automatic hematology analyzers currently in use electronically count blood cells by use of detectors which detect the difference in conductivity between the cells and the diluent. Such analyzers take advantage of the fact that blood cells are relatively poor conductors whereas the diluent is a relatively good electrical conductor.
In operation, a sample is drawn through a filter and passes through an aperture in a typical blood cell counter. Electrodes are generally located above and below this aperture and serve to sense the changed resistance as a blood cell passes the electrode sensing area. The electrodes are connected to appropriate circuitry which generates a pulse in response to each cell passing through the aperture. The pulses are electronically totaled over a predetermined period, and the results generated, for example, on a calibrated meter as millions of red blood cells per cubic millimeter, or as thousands of white blood cells per cubic millimeter. Counting devices having the above features can be found in commonly assigned U.S. Pat. Nos. 3,783,376; 3,812,425; 3,861,800; and 3,973,194.
Because of the great differences in concentration between red blood cells and white blood cells (WBC) in blood, different dilutions are necessary to obtain accurate counts. In addition, a lysing agent must be added to the dilution to remove red blood cells when a WBC count is taken. Systems which utilize only one counting head therefore require a complete flushing of the instrument when, for example, an RBC is to be taken after a WBC assay. Even with flushing, the lysing agent often has a residual effect which can introduce inaccuracy in the RBC. Furthermore, two separate tests are required to determine RBC and WBC in systems having only one counting head, and such tests cannot be conducted simultaneously. Because of the disparity in size between WBC and RBC or platelets, it has also been advisable to change the aperture size between counts. These procedures are all time consuming for the operator.
Present systems which have the fluidics and circuitry to perform RBC and WBC simultaneously are fully automated systems, such as disclosed in U.S. Pat. No. 3,549,994. An inexpensive counter, capable of analyzing prediluted and/or lysed blood samples so as to simultaneously determine RBC and WBC, has not yet been developed.